RNA tumor viruses are unique in several ways. They replicate through a DNA intermediate which is transcribed from virion RNA by viron reverse transcriptase. The virion RNA has a 'cap' on its 5' end and poly A on its 3' end. During reverse transcription in avian sarcoma virus-infected avian cells plus strand (virion RNA polarity synthesis begins before the completion of minus strand (complementary to virion RNA). We have been studying the fate of poly A in infected cells. Preliminary results indicate that as soon as plus strand synthesis begins in the cytoplasm of infected cells poly A seems to be removed. However, we do not know whether it is transcribed into poly dt. Experiments are in progress to delineate the mechanism of removal of poly A. We have also developed a simple method to rapidly isolate viral DNA from infected cell nuclei. This method selectively removes more than 98% of cellular DNA and allows complete purification of supercoiled viral DNA in less than a day or two. During the course of the development of this method we have also noticed that ASV DNA forms complexes, probably mini-chromosomes, with cellular proteins.